1. Field of the Invention
This invention relates to the field of well logging tools of the type wherein electromagnetic (xe2x80x9cEMxe2x80x9d) energy is used for measuring characteristics of formations surrounding a borehole. More particularly, this invention relates to an improved antenna coil shield for use in such tools to provide selective attenuation of the EM waves emitted or received by the antenna.
2. Description of Related Art
Induction and propagation well tools have been employed in logging operations for many years to measure the properties of subsurface formations surrounding an earth borehole. In conventional logging techniques, a number of antennae or coils are mounted on a well tool. An alternating current energizes one or more transmitter coils to emit EM energy into the formations. The emitted energy propagates through the formations or induces currents in the formations surrounding the borehole. The EM energy or currents are detected and measured by one or more receiver coils on the tool. The measured EM signals are processed to determine the electrical properties, such as permittivity or conductivity, of the formations.
If the transmitter and receiver coils on these tools were perfectly configured and balanced in a theoretically ideal system, the EM energy emitted by the coils would propagate in a mode known as a transverse electric (xe2x80x9cTExe2x80x9d) mode, of the type generated by an ideal vertical magnetic dipole in an azimuthally symmetric media. However, under actual operating conditions, there are various factors that give rise to the generation of significant undesired EM field components. One approach to alleviating this problem is with the use of antenna shields to reduce the transmission and/or reception of spurious and unwanted EM field components. These shields are typically used in conjunction with each coil on the tool.
U.S. Pat. Nos. 4,536,714 and 4,949,045 (both assigned to the assignee of the present disclosure) disclose conventional antenna shields employed in these well tools to provide mechanical protection for the coils and to guarantee the passage of desired EM field components. As shown in FIG. 1a, these shields 10 are in the form of a metal cylinder that has slots 12 in the axial direction. The slot 12 pattern allows the azimuthal electric field (Excfx86) component of the EM energy to pass, but prevents radial (Er) and axial (Ez) electric field components from passing through the shield, either from within (in the case of a transmitter) or from without (in the case of a receiver). An alternative viewpoint is to represent each axial slot 12 as an axial magnetic dipole, as shown in FIG. 1b. These magnetic dipoles are sensitive to axial magnetic fields (Bz), but they are not sensitive to azimuthal magnetic (Bxcfx86) fields. The shielded coils are thus rendered insensitive to parasitic transverse magnetic (xe2x80x9cTMxe2x80x9d) EM fields associated with borehole modes, and which have radial (Er) and axial (Ez) electric fields and azimuthal magnetic fields (Bxcfx86).
Recent publications in the field of well logging have described the implementation of tools with triaxial coils. Such coil configurations involve three coils with magnetic moments that are not co-planar. U.S. Pats. Nos. 5,508,616, 5,115,198, 5,757,191 and PCT Application WO 98/00733, Bear et al., describe logging tools employing such coil configurations. Common to these apparatus and techniques, however, is the need to manipulate the antenna coil itself. None of these disclosures address the implementation of antenna shields as alternative means to achieve selective EM energy attenuation.
It is desirable to rotate the axis of the magnetic dipole of a transmitter or receiver coil without having to tilt the axis of the coil in relation to the tool axis. The benefits of such a technique include reductions in manufacturing and re-tooling costs, as well as shorter production times. Still further, it is desired to implement a shield apparatus that can be used in conjunction with tilted and non-tilted coils to rotate the axis of the magnetic dipole.
A shield apparatus adapted for use in conjunction with a well tool is provided to selectively attenuate one or more electromagnetic energy field components as the components interact with the shield.
In a first aspect of the invention, a shield containing a sloped slot pattern is provided to surround a coil. The shield attenuates selected electromagnetic energy field components as they interact with the shield to pass the desired components and restrict unwanted components.
In a second aspect of the invention, a strip is provided to surround a coil. The strip contains conductive elements configured in a sloped pattern. The strip thereby attenuates selected electromagnetic energy field components as they interact with the shield to pass the desired components and restrict unwanted components.
In a third aspect of the invention, a method for rotating the axis of the magnetic dipole of a transmitter or receiver coil is provided.
In a fourth aspect of the invention, a method for winding and shielding an electric coil is provided. The resulting coil emits or receives selected electromagnetic energy field components.